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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b="Uf3p/qK8"; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9F93F606A332 for ; Thu, 5 Nov 2026 14:36:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=Uf3p/qK8 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 552531F00A02; Thu, 5 Nov 2026 14:36:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793889390; bh=WNaqt/zIb1JcNg4K9VF2Y1BZCn/3lY00rDpw9JPXaO4=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Uf3p/qK8HfA39sBKpscrP9nuzc7KDsHZ2Vx8Y9W96aHVWtCxznsy40xG0npgM9465 5zwUW0irw1IHHwMfWjgQTbTMZcKc6P6628L0oLGw+0keqR0HR52xr0xgmGePiiJECZ NwHotTeojiTcb3NGGIa2fHVQZXsnCR88mG5sxv2zZLh9D2b8ZfFqUaU+ZHJb/9pXCN 5NKgc/goLiGEmoilpEqvIGFiFwenpnRpLZ/LdGvzQu0msxmMz2RUAYwxhdI6TWb39x KDDm58Kbn3YpOFvGPgIeGwvGXOrmATQf3G8oifQdLXSdW1OjmULR3C+QhV+3Fm+bkY pGcQ56QvG3v2w== Message-ID: <7ac409a1-fe3e-4b37-8130-4d49ecb6106a@kernel.org> Date: Thu, 5 Nov 2026 15:36:25 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 04/10] mm, slab: introduce slab_debug=N Content-Language: en-US To: Harry Yoo Cc: Andrey Ryabinin , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , Geert Uytterhoeven , Conor Dooley , Damien Le Moal , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Alexander Potapenko , Andrey Konovalov , Dmitry Vyukov , Vincenzo Frascino , kasan-dev@googlegroups.com References: <20261105-slub_tiny_rework-v3-0-0484ce59eabb@kernel.org> <20261105-slub_tiny_rework-v3-4-0484ce59eabb@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/5/26 15:30, Harry Yoo wrote: > On Thu, Nov 05, 2026 at 09:43:53AM +0100, Vlastimil Babka (SUSE) wrote: >> Enabling slab_debug has the side-effect of disabling all percpu caching >> of objects (now via sheaves), which can be sometimes useful for saving >> memory for e.g. kdump kernels. To make this possible without the >> overhead (CPU or memory) of actual debugging options such as poisoning, >> introduce a No-op debug option that only forces the debugging slow paths >> and zero sheaf capacity. The new corresponding slab flag is >> SLAB_DEBUG_NOOP. >> >> This will also allow reimplementing CONFIG_SLUB_TINY as a boot-time >> option. >> >> Two details have to be considered, as suggested by Harry Yoo [1]. >> >> - While SLAB_DEBUG_NOOP is part of SLAB_DEBUG_FLAGS, it should not >> prevent cache merging as it doesn't affect object metadata, and lack >> of merging would go against the memory footprint reduction. Thus >> exclude the flag from SLAB_NEVER_MERGE. >> >> - However add the flag to SLAB_MERGE_SAME, per sashiko. >> >> - SLAB_DEBUG_NOOP alone should not request disabling printk pointer >> hashing. Thus rework the call to hash_pointers_finalize() to not rely >> on the slub_debug static key state. Instead determine whether the >> global or per-cache slab_debug options include any flags that are not >> SLAB_DEBUG_NOOP. This does not change the fact that caches created >> with explicit debug flags (i.e. slub_kunit tests) later in boot have >> no effect on pointer hashing, which is determined in >> kmem_cache_init(). >> >> While documenting the new N option, remove obsolete SLAB references from >> the F option description. >> >> [1] https://lore.kernel.org/all/arUx56ee_nkxYP_f@thinkstation/ >> [2] https://sashiko.dev/#/patchset/20261024-slub_tiny_rework-v2-0-bf1419ab5ffd@kernel.org?part=2 >> >> Signed-off-by: Vlastimil Babka (SUSE) >> --- > > With the sashiko report to this patch fixed, this looks good to me. > Reviewed-by: Harry Yoo (Meta) > > One nit comment: > > specifying SLAB_DEBUG_FLAGS depends on CONFIG_SLUB_DEBUG, and with this > patchset there is no way to minimize slab memory usage w/o SLUB_DEBUG. Maybe not at this point, but unless I made some mistake, later slab_tiny=1 stops depending on SLUB_DEBUG. slab_debug=N itself does depend on it but I think that's acceptable > Sounds like it's time to make SLUB_DEBUG unconditional. It's enabled on > all distros listed in kconfigs [1]. Yes it's my plan to do that next, I did even discuss it with Christoph at LPC :) > I built kernels w/o SLUB_DEBUG and the difference in the kernel > binary was about 40 KiB. > > [1] https://oracle.github.io/kconfigs/?config=UTS_RELEASE&config=SLUB_DEBUG >